Potamonautes lirrangensis – Malawi Blue Crab
Potamonautes lirrangensis was described in 1904 by Mary J. Rathbun, then as Potamon lirrangensis. In 1907 Cunnington made a description of Potamon orbitospinus. In retrospect, these were one and the same species.
At many stores you often see the old name: Potamonautus orbitospinus. This is a synonym that shouldn’t be used anymore. Since Potamonautes lirrangensis has been described earlier, this is the official name and Potamonautus orbitospinus has become a synonym.
Their common name is Malawi Blue Krab.
The carapace of Potamonautes lirrangensis can grow to about 10 centimeters wide. Including the legs they are about 15 centimeters wide and 20 centimeters if you spread the legs out completely. The color of the Malawi Blue Crab does not have to be blue. You often see the blue version in stores. The color varies from brown to blue. A red color can be seen on the joints.
The difference between the male and female is clearly visible in the abdomen. In the woman it is much wider than that of the man. In fact, that’s the crab’s tail. This is folded under the body. The female keeps the eggs between this tail and the lower body. Here they are safe from predators.
Potamonautes lirrangensis is best kept solitary. They usually do not tolerate conspecifics in the neighborhood. Even a 150 centimeter long aquarium with enough hiding places is often not big enough.
Malawi Blue Crabs only come together during mating. But even then, the female may be killed by the male.
The Malawi Blue Krab is not only found in Lake Malawi. You can also find them in Lake Kivu, Congo and Malagarasi rivers. They are not endangered in the wild, partly due to the large distribution area where they occur.
Like many crabs, the Malawi Blue Crab is omnivorous. They eat just about anything edible. You can feed them with vegetables, fruit, sinking granulate, wafers, pieces of fish, shrimp, mussel, snails, etc.
They don’t normally eat live fish. Only sick and weak animals that can no longer swim away are eaten. They are scavengers and not hunters.
As mentioned, it is best to keep Potamonautes lirrangensis solitary. If you only keep one crab you can keep them in an aquarium from about 100 liters. However, they are usually kept in a Malawi aquarium. The minimum size should then be about 150 centimeters in length.
Decorate the aquarium with sand on the bottom. Place rocks in such a way that there are holes and crevices for the crab to hide between. Make sure the rocks are placed really stable so that they don’t move when the crab crawls between them.
You can keep the Malawi Blue Crab with not too big fish species. When a crab sheds its new armor is still soft. Predator species that are too big can then attack and eat the crab. Losing your crab this way would be a real shame.
In some descriptions you can see that this crab needs a bit of dry land to climb out of the water. That is not the case. In Lake Malawi, for example, they live to a depth of about 50 meters. These crabs never surface. But if you can provide them with a piece of dry land, they will use that for short periods of time. They will climb out of the water from time to time.
Note that the Malawi Blue Crab is a true escape artist. They can climb very well. An open aquarium is therefore not suitable for this species. Make sure that the aquarium or paludarium has a tight-fitting lid!
Breeding the Malawi Blue Crab
Breeding in the aquarium is difficult. As mentioned, Potamonautes lirrangensis does not tolerate other crabs in their vicinity. They do not hesitate to kill and eat conspecifics. This can even occur in an aquarium of 8,000 liters.
After mating, the female cares for and guards the eggs and young crabs under her body. As soon as the young crabs leave the mother, they already look like the adults, of course much smaller. A single nest can yield up to a few hundred young crabs.
It is observed in wild caught crabs the female can be fertilised and preserve the semen for months. Wild caught females can fertilise their eggs months after they have been caught.
John de Lange
Rathbun, M.J. (1904). Les crabes d’eau douce (Potamonidae). Nouvelles Archives du Muséum d’Historie naturelle. Nouvelles Archives du Muséum d’Historie naturelle, series 4. 6: 225-312.